Melanoma patients who responded to the combination of the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) blocker ipilimumab and the PD-1 blocker nivolumab had a gut microbiome with increased abundances of F. prausnitzii, Bacteroides thetaiotamicron, and Holdemania filiformis, whereas patients who responded to the PD-1 blocker pembrolizumab had a gut microbiome enriched in Dorea formicogenerans. The gene discussed is PDCD1; the disease is melanoma.